Lignin is a natural polymer, which can be extracted from e.g. wood. As lignin is a natural biopolymer its use as a component in e.g. glues instead of synthetic materials has been investigated in order to come up with more environmentally friendly applications.
Lignin can be separated from e.g. wood during e.g. a pulping process during which lignin is delignificated and dissolved from the wood matrix into a cooking liquor. Different methods can be used for recovering lignin from the cooking liquor. For example, lignin can be precipitated from the cooking liquor by using an acidic reagent, such as CO2 or sulphuric acid, after which the precipitated lignin can be separated by e.g. filtration. Attempts have also been made to fractionate lignin according to the molecular weight of lignin in order to recover lignin with specific properties to be used in specific applications. For example ultrafiltration, nanofiltration and sequential precipitation have been used for fractionating lignin.
Lignin can be precipitated from black liquor by lowering the pH e.g. from 13-14 to 8-9. However, low molecular weight lignin is not precipitated in such pH values but stays in the mother liquor. When the precipitated lignin is separated from the remaining liquor, the low molecular weight lignin will end up in a waste filtrate with cooking chemicals and impurities. The waste filtrate is often circulated to an evaporation plant and burned in order to recover cooking chemicals.
Low molecular weight lignin can be recovered from e.g. kraft lignin by ultrafiltration. However, drawbacks of the ultrafiltration process are a rather low yield and an expensive procedure.
The inventors have therefore recognized a need for an effective method for recovering also low molecular weight lignin from black liquor.